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Old Oct 5, 2010, 10:34 AM   #1
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Default ISO vs. shutter speed comparison

The other day I was comparing my T1i with 18-55 kit lens to a friend with a Nikon D40 with 18-55 lens. We were messing around with the settings, and when they were both on ISO 400, the T1i chose a shutter speed of around 1/10 - 1/20 and the Nikon chose between 1/40-1/50. I am just curious is the lens or the camera. Whenever we changed the ISO, the Nikon seemed to be able to get a faster shutter speed. This was on P mode and they both used the same aperture.
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Old Oct 5, 2010, 10:42 AM   #2
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Hmm...I would have to assume something else changed. Best way to compare is to look at the output of both and see the exif data on the shots. You never know what may transpire in P mode as the camera has control of the helm. This is why I always shoot full manual.
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Old Oct 5, 2010, 10:59 AM   #3
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... they were both on ISO 400, the T1i chose a shutter speed of around 1/10 - 1/20 and the Nikon chose between 1/40-1/50. ...
How did you find out what the shutter speed was? If you had looked at the EXIF data you would know what it was exactly, and you would know for sure that the aperature and ISO were the same and that the flash didn't fire on one of them.
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Old Oct 5, 2010, 11:19 AM   #4
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Just when half pressing the shutter and the camera shows what shutter speed it is choosing. Is it not common that different cameras would choose different shutter speeds for the same ISO settings? Does it have to do with the quality of the camera/lens? I would think though in this case, the lenses would be similar quality, but am not familiar with the Nikon lens.
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Old Oct 5, 2010, 11:38 AM   #5
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You left out the third factor in determining exposure - Aperture. Try repeating with the aperture set the same on both cameras.

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Old Oct 5, 2010, 11:42 AM   #6
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No, I said they used the same aperture. So in P mode, half pressing the shutter, both cameras chose the same aperture, which was the most wide open for that focal length. So in full zoom, it was at 5.6. Then they chose the shutter speeds I listed. So it is sounding like they should have been the same? Are ISO speeds the same on all cameras? I will have to do a better comparison next time I see him.

I also notice when I have the T1i on auto ISO, it seems to choose 800 when I am using the flash, so I just manually set it to 100. Why would it choose such a high ISO when it is using the flash?
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Old Oct 5, 2010, 11:57 AM   #7
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Actual sensitivity varies somewhat among cameras, so an ISO setting of 800 may be nearly a full stop different. The actual shutter speeds may not be exactly what is displayed, either. As BillDrew mentioned, look at the Exif data, to find what the actual numbers are. Also, note if either of you had adjusted the Exposure Compensation, that would have changed the exposure settings.

Frankly, I hate when the camera starts picking its own ISO setting. Not real fond of getting the aperture changed on me either. As you might guess, I use manual settings mostly. That way, I have only myself to blame if (OK, when) something goes wrong. Saves wear and tear on the camera and lenses when they don't get thrown into brick walls.

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Old Oct 5, 2010, 12:02 PM   #8
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I also notice when I have the T1i on auto ISO, it seems to choose 800 when I am using the flash, so I just manually set it to 100. Why would it choose such a high ISO when it is using the flash?
Your flash has a guide number that should be in your manual. The GN will be in either feet or meters for a specified ISO. Let us imagine that your camera's built-in flash has a GN of 64 ft at ISO 100. To use that number, divide the GN by the aperture you are setting. At f/4, your flash would be able to properly illuminate an object that was as much as 64/4, or 16, feet away.

Now, if you change the ISO that you use, you also change the GN for your flash unit. Every time you quadruple the ISO setting, you double the GN. So, for your camera's 64 GN at 100 ISO, you have a GN of 256 at 1600 ISO. By increasing the ISO, the camera increases the maximum reach (or the minimum aperture) that can give proper exposures.

Last edited by tclune; Oct 6, 2010 at 10:44 AM. Reason: fix math stupidity
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Old Oct 5, 2010, 1:33 PM   #9
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Hey thank you, that is very helpful. I did not know about the guide number and stuff. I found my flash's GN is 43 feet. So at full wide the flash can cover 43/3.5 = about 12 feet at ISO 100. So at ISO 400, it can cover 24 feet? And at ISO 1600, 48 feet? Really? I didnt realize a flash could cover that far. So I guess if I am just doing closeup shots of people within 12 feet, I am better off leaving it at ISO 100. But if I go full zoom at 55mm and then it is at F5.6, that would only cover 7.6 feet so I would either have to get closer, or bump it to ISO 400 to get 15 feet. I will have to pay attention to what the auto ISO does depending on how far away I am.
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Old Oct 6, 2010, 10:50 AM   #10
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There is another advantage of increasing the ISO, even for relatively close shots -- the less power that the flash requires to illuminate a subject, the faster its refresh rate can be. I know a lot of people believe that you should always use the lowest ISO rating you can get away with because it will have the lowest noise. But I tend to favor using the highest ISO rating that you routinely find the noise to be unobjectionable for because of the other advantages that accrue to it, as with using flash. You just have to decide for youself what you're willing to give up and what you want to maximize.
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